1. Field of the Invention
This invention relates to the detection of hazardous and/or toxic gases, including warfare gases, and more particularly to a method and apparatus for detecting such gases using optical systems such as laser photoacoustic spectroscopy (LPAS).
2. Description of the Related Art
The terrorist events of Sep. 11, 2001, subsequent anthrax mailings, and the 1995 Tokyo subway Sarin attack by the Aum Shinrikyo cult has heightened worldwide awareness of catastrophic social impact of potential large scale attacks using chemical warfare agents (CWAs), and has exposed the critical need for the reliable, unambiguous and early detection of trace CWAs and toxic industrial chemicals (TICs) in the air. Despite the fact that all countries worldwide are signatories to the Chemical Weapons Convention, which bans the use of CWAs, the U.S. defense establishments have considered their use possible and thus have developed battlefield sensors for the ambient detection of CWAs to protect troops. However, CWA sensors suitable for civilian use in places such as airports, railroads stations, large public and private office buildings, theaters, sports arenas, etc., have received much less attention. Such civilian sensors may require different performance than those deployed in battlefields.
For example, in public settings there is a need for the early detection of CWAs so that parts of buildings can be isolated or evacuated, while a low probability of false positives (PFP) is a necessity to avoid the adverse economic impact caused by false alarms leading to unnecessary evacuations.
A brief background on photoacoustic IR spectroscopy begins with the observation that IR absorption spectroscopy is a powerful tool for trace gas detection because a vast majority of polyatomic molecules, including CWAs, TICs and explosives, absorb light in the wavelength region from 3 to 14 μm. FIG. 1 shows the IR spectra of nerve gases, mustards gases, and TNT, illustrating that the most prominent features for many species of interest lie between 3 and 11.5 μm. Table 1 lists some of the pertinent species that can be detected in different wavelength regions.
TABLE 1CWAs, TICs and Explosives that can beDetected in Specific Spectral Regions9-11.5 μmCWAsLewisite, Nitrogen Mustard (H—N3), Sulfur mustard(HD), 4-Dithiane, Diisopropyl methylphosphonate (DIMP),Dimethyl methylphosphonate (DMMP), Isoamyl alcohol,Methylphosphonic difluoride (DIFLUOR), Cyclosarin (GF),Sarin (GB), Soman (GD), Tabun (GA), VX, Triethylphosphate, 2-diisopropylaminoethanol (DIPAE)TICsAmmonia, Arsine, Boron trichloride, Ethylene oxide,Nitric acid4-9 μmCWAsMustard (H—N3), Sulfur mustard (HD), 4-Dithiane,TICSBoron Trifluoride, Carbon Disulfide, Diborane,Formaldehyde, Hydrogen Cyanide, Hydrogen Sulfide,Nitric Acid, Phosgene, Sulfur Dioxide, TungstenHexafluorideExplosivesTNT, PETN2.5-4 μmTICsHBr, HCl, HF
When evaluating an optical detection technology, certain performance characteristics are important and include: sensitivity, specificity, probability of false positives (PFPs) probability of false negatives (PFNs), response time, and recovery time.
Detection sensitivity is a key indicator of overall sensor performance and relates to the minimum gas concentrations that are reliably detected. A good sensitivity enables detecting CWAs or TICs before the concentration rises to dangerous levels, monitoring at low levels for long term exposure problems, and determining when an attack site is safe to reenter.
Specificity, i.e., ability to distinguish between different CWAs, is important to first responders in order to provide appropriate treatments subsequent to the exposure. Specificity yields information not only about how much of a toxic gas is present, but also which gas is present.
Probability of false positives (PFP) provides a number that represents the fraction of measurements that falsely indicate a toxic gas is present when in reality it is not. Such false alarms often arise from interfering gases that might also be present in the indoor or outdoor environment and typically represent the most significant operational difficulty for field-deployed sensors. Very low PFP is desirable since false alarms cause substantial and expensive disruption in the normal routine of those at the measurement site.
Probability of false negatives (PFN) produces a number that reflects the fraction of measurements that falsely indicate a toxic gas is not present even though it is present at or above the set threshold level. A low PFN is desired in order to prevent unknown exposure to toxic air.
Response time relates to near real-time functionality (response time≦60 seconds) which is necessary to provide warnings that are useful for protecting people and evacuating attack sites.
The recovery time parameter reflects the time that a sensor requires to recover from a high reading, whether after an exposure to CWAs and/or TICs or a false reading prompted by the presence of an interfering gas.
Designing a sensor to simultaneously satisfy these standards requires a quantitative understanding of the sensor's operational characteristics as well as the nature of interferences expected in realistic environment and their impact on the sensor's operational characteristics.
The required sensitivity for CWA detection can be determined by the toxicity levels of particular agents, most of which have been reasonably well documented. With the 1995 Tokyo scare, Sarin (GB), a typical nerve agent CWA, has received interest within the scientific community; the concentrations and related health effects for it are summarized in Table 2.
TABLE 2Summary of Allowable Sarin Dose for Different Health EffectsHealth EffectDoseRemarksLethal (50%)100mg-min/m3RestingIncapacitation (50%)75mg-min/m3RestingMiosis1mg-min/m3Occupational Limit48μg-min/m38 hour/day, 40 hours/week, 40 yearsGeneral Population12.96μg-min/m3
For a 30 minute exposure to Sarin, the lethal concentration is approximately 575 parts-per-billion (ppb), or 3.3 mg/m3, while the first noticeable health effect (miosis) occurs at 33.3 μg/m3 (5.8 ppb). For the general population, the suggested limit would be 27 ng/m3 (4.7 ppt) for an 8 hour exposure. Thus, to protect population from harm in the event of a Sarin attack, reasonable design targets for CWA sensors include 1) a sensitivity of approximately 1 ppb (well below the harm threshold for 30-minute exposure), and 2) a measurement time shorter than 1 minute to allow reasonable time for evacuation. The thresholds for harm from the other CWAs are similar.
The CWAs have absorption strengths of ˜3-6×10−3 [ppm.meter]−1 (FIG. 1). For illustration, VX may be used as a typical CWA, having a peak absorption of 2×10−3 [ppm.meter]−1 at 9.6 μm. Consequently, the infrared absorption at 9.6 μm from 1 ppb of VX would be 2×10−8 cm−1. Thus, detecting typical CWAs at ppb and sub-ppb levels using optical absorption techniques requires an absorption measurement capability as low as 10−8 cm−1.
Though sensitivity is a key parameter for a trace gas detection sensor, it is the selectivity (the ability to discriminate the target from interferences to avoid false alarms) that becomes the limiting performance factor when the sensors are used in real-world settings. In these environments, the ambient air can often be heavily contaminated with potential interferences.
Traditionally, absorption spectroscopy sensors avoid the effect of interferences by measuring absorption either at a single wavelength or over narrow wavelength region where the target gas absorbs, but the other gases in the sample do not absorb. This approach has been successful for the detection of smaller molecules in relatively clean samples, where the target spectra are sharp and the potential interferences are minimal. However, because CWAs and a majority of interferences that occur in realistic air samples are relatively large polyatomic molecules, their IR spectra are characterized by broad absorption features as seen from FIG. 2 that shows the infrared absorption spectrum of VX and an interference, butyl acetate. The spectra are several hundred nanometers wide, which is typical of CWAs and interferences, and overlap significantly between 9.5 and 9.9 μm. Since the targets and interferences absorb the probe light at many of the same wavelengths, selective spectroscopic detection of the target requires the acquisition of the spectrum over a broad wavelength range followed by quantitative decomposition into the contributing spectral signatures of the targets and interferences. Thus absorption sensors that operate over a narrow wavelength range would not be able to adequately distinguish these two species.
A sensor's selectivity is typically explained quantitatively through the PFP (or false alarm rate). For a sensor that has a one-minute measurement time, a reasonable design target is to achieve a PFP less than 2×10−6, which corresponds to a false alarm rate of <1 per year. The design targets for 10 second and 1 second measurement times would be a PFP of 3×10−7 and 3×10−8, respectively in order to keep the false alarm rate at less than 1 (one) per year.
As described above, sensors need to detect absorptions as small as 10−8 cm−1. There are a number of optical techniques that permit measurements of such small absorptions, the most common of which are long path absorption measurements (e.g., multipass cells and cavity ring-down spectroscopy) and calorimetric techniques (e.g., photoacoustics).